Being a first generation antihistamine, it has all the associated problems such as drowsiness and dry mouth. These are some of the least specific small molecule therapeutics you could hope for, which gives rise to many of the side effects. Interestingly, it has some effect on serotonin reuptake, which led to some drugs for depression with similar structure.

“Interestingly, it has some effect on serotonin reuptake, which led to some drugs for depression with similar structure.”

One of the things I learned in the course of trying (more or less successfully) to find the right meds for my chronic depression is that some doctors will prescribe diphenhydramine as a “booster” for “regular” antidepressants. I’ve known a couple of people who have taken it for exactly that reason.